1. Field of the Invention
The present invention relates to an internal combustion engine control technology, and more particularly to an internal combustion engine control technology that is suitable for controlling an idling speed during a cold start.
2. Background Art
During a cold start, the rotation speed of an internal combustion engine is likely to differ from a target rotation speed. Therefore, various technologies were proposed for controlling an idling speed during a cold start. The technology disclosed, for instance, by Japanese Patent No. 2505304 (hereinafter referred to as “Patent Document 1”) inhibits the rotation variation of an internal combustion engine during a cold start. The technology described in Patent Document 1 detects the rotation variation of each cylinder during idling. If an upper limit value is exceeded by the rotation variation of a certain cylinder, this technology decreases an injection amount for the cylinder and increases the injection amount for the other cylinders. If, on the other hand, a lower limit value is exceeded by the rotation variation of a certain cylinder, this technology increases the injection amount for the cylinder and decreases the injection amount for the other cylinders.
The difference between the actual rotation speed and target rotation speed of an internal combustion engine during a cold start is attributable to various causes. One cause is a friction change with time, a temporary increase in the air-conditioner load or other electrical load, or a manufacturing error such as the flow rate variation of a throttle system. Another cause is the use of heavy fuel. If the former causes exist, the intake air amount deviates from its target value no matter whether the combustion state prevailing within the internal combustion engine is good. As a result, the actual rotation speed deviates from the target rotation speed. If, on the other hand, the latter cause exists, the air-fuel ratio is likely to become lean because the heavy fuel is more unlikely to evaporate than the regular fuel. As a result, the rotation speed varies due to combustion state degradation such as irregular combustion or engine flameout, causing the actual rotation speed to differ from the target rotation speed. To assure stable idle running, it is necessary to control the internal combustion engine in such a manner as to eliminate the difference between the actual rotation speed and target rotation speed. It is believed that the optimum control method varies depending on whether the combustion state is good or not.
However, the conventional technology is not concerned with the cause of the difference between the actual rotation speed and the target rotation speed for idling speed control. The technology disclosed, for instance, by Patent Document 1 corrects the fuel injection amount in accordance with the degree of rotation variation and without regard to the cause of rotation variation. However, if rotation variation arises out of the use of heavy fuel, the technology adds a considerable amount of fuel, thereby incurring exhaust emission deterioration. To efficiently eliminate the difference between the actual rotation speed and target rotation speed while avoiding such exhaust emission deterioration, it is necessary to employ an optimum control method in accordance with the cause of rotation variation.